1. Field of the Invention
The present invention generally relates to a device for obtaining signals representative of a relative speed of a vehicle with respect of an ambient fluid, and more particular to such a device employing fringe laser anemometers.
2. Discussion of the Background
To direct some moving vehicles, such as aircraft or some missiles, and even boats, the knowledge of their relative speed with respect to an ambient fluid is necessary. While a aircraft will be discussed below, it is understood that the following discussion is also valid, except for the type of fluid, for other types of vehicles whether they fly or move on the ground, on the water or under the water.
It is known, in an aircraft, to determine the relative speed of aircraft with respect to an ambient fluid from the signals provided by four probes including:
a static pressure probe whose sensitive surface is placed parallel to the airflow streams at a location where neither partial vacuums nor excess pressures occur,
a total pressure probe whose sensitive surface is placed generally to the front of the aircraft, at least approximately perpendicular to its longitudinal axis,
two clinometric probes, for example of the weather vane type, which give respectively the angle of incidence and the sideslip angle of the aircraft. As used here, the angle of incidence is defined as the angle that the relative speed vector makes relative to the wind with the chord of the wing of the aircraft, i.e. with the line which connects the foremost point and the rearmost point of the wing in a vertical section parallel to the plane of symmetry of the aircraft. Additionally, the sideslip angle is defined as the angle that the relative speed vector makes relative to the wind with the longitudinal axis of the aircraft. The knowledge of the angle of incidence is vital because it makes it possible to prevent the risks of stall.
These four probes form protuberances on an outside wall of the aircraft and as a result present various drawbacks. For example, the use of probes poses problems of mechanical strength in bad weather, as well as problems with the correct operation of the probes in any weather, in particular in icy weather. Additionally, the use of probes makes an aircraft more easily detectable by radar, which is a difficulty for the military aircraft. Further, the probes require, as a function of the angle of incidence, correction of the measurements made by the pressure probes. Moreover, the use of probes creates turbulence which increases drag.